(a) Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating the same, and more particularly to a so-called buried semiconductor laser device having a higher laser emission efficiency and a higher reproducibility of a current-optical output characteristic.
(b) Description of the Related Art
A semiconductor laser device having a lower threshold current density and a higher laser emission efficiency is desirable. A strained quantum well semiconductor laser device having a hetero-structure and a pair of current blocking layers is attracting public attention because of the excellent characteristics thereof. The semiconductor laser having a pair of current blocking layers in abutment to the semiconductor laser structure is generally called a buried semiconductor laser.
A conventional strained quantum well semiconductor laser device shown in JP-A-8(1996)-288589 will be described referring to FIG. 1A.
As shown in idealized form in FIG. 1A, a conventional strained quantum-well semiconductor laser device 20 includes a layer structure having an n-type InGaP bottom cladding layer 2, an active layer 3, and a p-type InGaP top cladding layer 4, sequentially and epitaxially grown on an n-type GaAs substrate 1 by using a metal organic chemical vapor deposition (MOCVD) method.
The active layer 3 is a five-layered structure including an InGaAsP layer 5, a GaAs layer 6, an InGaAs layer 7, a GaAs layer 8 and an InGaAsP layer 9.
The top cladding layer 4, the active layer 3 and the top part of the bottom cladding layer 2 are configured to have a mesa structure 11. Each of the side surfaces 12 of the mesa structure 11 and the adjacent surfaces of the bottom cladding layer 2 are covered with a p-type InGaP current blocking layer 14 and an n-type InGaP current blocking layer 15, which are sequentially deposited.
A second p-type InGaP top cladding layer 16 and a p-type contact layer 17 are sequentially deposited on the n-type InGaP current blocking layer 15, the p-type InGaP current blocking layer 14 and the top cladding layer 4 of the mesa structure 11.
A p-side metal electrode layer 18 and an n-side metal electrode layer 19 are deposited on the top surface of the p-type contact layer 17 and the bottom surface of the substrate 1, respectively.
The above publication points out a problem when the p-type current blocking layer 14 and the n-type current blocking layer 15 are grown by using an etching mask. Referring to FIG. 1B, structural defects such as hollows and grooves 40 are formed on the n-type current blocking layer 15 along the bottom surface of the etching mask due to the difference between the growth rates.
When the hollows 40 on the n-type current blocking layer 15 are large, crystal dislocations are liable to occur along the lines 41 shown in FIG. 1B. The propagation of a crystal dislocation from a point within layer 15 to a point within the p-type contact layer 17 increases the threshold current of the fabricated laser device, which lowers the laser emission efficiency.
The above publication describes the growth conditions of the p-type and n-type current blocking layers 14, 15 such that the substrate temperature is between 750° C. and 800° C. and a mixing ratio (concentration ratio) of a group V element gas with respect to a group III element gas is between 400:1 and 800:1 inclusive (V:III), thereby suppressing the occurrence of the structural defects (e.g., hollows) to decrease the probability and magnitude of the crystal dislocations. (As used later herein, we will abbreviate the conventional notation for the V:III chemical ratios from 400:1 to simply read as “400,” which means the molar amount of the group V element gas divided by the molar amount of group III element gas).
Since the disappearance of the structural defects thickens the n-type current blocking layer 15 in the vertical direction formed overlying the substrate 1, the amount of leakage current flowing through the current blocking layers 14, 15 is decreased, which in turn increases the laser emission efficiency when a voltage is applied between the electrodes 18, 19.
Further, Mitsubishi Denki Giho (Mitsubishi Electric Advance) Vol. 67, No. 8 (1993), p. 88 points out a decrease of the laser emission efficiency due to a leakage current which does not contribute to the laser emission and which flows along the interface between the mesa structure and the current blocking layer.
The buried semiconductor laser device with the reduced leakage current includes higher laser emission efficiency, good linearities of the higher output characteristic, and an excellent current-voltage characteristic. Accordingly, when the leakage current path width is reduced, the resistance of the current blocking layer increases to provide desirable laser characteristics.
Even when the current blocking layer is formed under the conditions described in the former publication such that the substrate temperature is between 750° C. and 800° C., and the mixing ratio between the group V element gas and the group III element gas is between 400 and 800, the leakage current path width is quite difficult to be formed in a narrower manner with the excellent reproducibility, and the values of the widths are difficult to be regulated and controlled.
Similarly, in the fabrication of the buried semiconductor laser device formed on the p-type substrate, an n-type InP contact layer is excessively grown to be in contact with an n-type InP contact layer, and a leakage current path width is increased.
As a result, the increased leakage current lowers the laser emission efficiency to worsen the output characteristic and the linearity of the current-voltage characteristic, and the buried semiconductor laser device with the higher output can be hardly fabricated with the excellent reproducibility.